In processing charge retaining webs, static electric charges tend to build up on the web. Typically, such webs have layers of somewhat conductive material (i.e. material having a surface resistivity of up to 1.times.10.sup.12 ohms, more particularly 1.times.10.sup.7 to 1.times.10.sup.12 ohms, measured between parallel edges of a square piece of that material regardless of size). During processing, these webs are charged by frictional contact with stationary guide surfaces in the web handling apparatus or by roller contact electrification. Such static charge build-up is found, for example, in webs of photographic products, such as photographic paper or film.
The build-up of static charge can result in a variety of serious problems. One problem caused by such charge is that the web material is attracted to itself or to handling apparatus. As a result, the operating efficiency of the apparatus is diminished. In addition, the presence of static charge attracts dust to the web surface and produces electrical discharges which can shock operators and cause undesirable exposure of photosensitive materials.
Static electric charges, present in and on sheets or webs, have been dissipated in a variety of ways.
In U.S. Pat. No. 3,757,164 to Binkowski, brush-like attachments are placed across a sheet or web in a contacting relationship to conduct charges to ground. Such contact can, however, scratch, mar, and wear down the sheet or web. Even if this device is used as a non-contacting, inductive neutralizer, the web is likely to be damaged by brush bristles which break off and fall onto the web. These brush filaments carried by the web can also interfere with the operation of web handling equipment.
In U.S. Pat. No. 3,533,692 to Blanchette et al. ("Blanchette"), an edge contacting device is utilized to maintain a photoconductive web at a desired potential. Such contact is, however, undesirable, because the web edge may be fragile.
Other devices use a power source to generate ions in an electric field capable of neutralizing static electrical charges. These devices may utilize direct current power sources, as in Blanchette, or corona discharge devices, as in U.S. Pat. No. 3,620,614 to Gunto et al., or alternating current power sources, as in U.S. Pat. No. 4,363,070 to Kisler. However, these devices are expensive to purchase and operate and are difficult to install compared with devices using no external power source.
Non-contacting devices have also been used to remove electric charges from a moving web. U.S. Pat. No. 3,268,766 to Amos discloses a device utilizing a system which forces air through a stationary guide in the web handling apparatus to provide a blanket of air between the web and a porous conductive surface. The guide is connected to a ground attachment, providing a path for discharge of the electrostatic charges. Unfortunately, this apparatus requires a source of compressed air which, again, increases installation and maintenance costs of the device and makes it less versatile relative to placement in the device.